Reverse osmosis systems have long been used for purifying water from salt-contaminated wells, cleaning industrial wastes, and the like, wherein salt concentrations of less than 5,000 parts per million (ppm) are present. Because of the low concentrations involved, the required pressure also is low, on the order of 200-300 pounds per square inch (psi), and the recovery (i.e. percentage of brine pumped which is converted to fresh water) is high, on the order of 90-95 percent; only a small amount of concentrated brine being needed to carry away the salts rejected by the reverse osmosis membrane.
Seawater reverse osmosis desalination is known, of course, which involves obtaining fresh water from seawater in which the salt concentration is on the order of say 35,000 to 40,000 ppm. Now, the required pressure is on the order of 800-1000 psi, and recovery is no greater than about 30 percent before the concentration of the brine exceeds the solubility levels for some of its constituents. Some pretreatment of the seawater is required for a 30 percent recovery; the 30 percent figure representing an optional balance between the costs of pretreatment and the present costs of pumping. The disposal of 70 percent of the high pressure brine from the reverse osmosis device represents a significant energy loss, and requires a significant investment in pumping equipment.
Energy recovery means for recovering energy from the high pressure spent brine discharged from reverse osmosis systems are known as shown, for example, in U.S. Pat. No. 3,825,122 to Taylor, Federal Republic of Germany Pat. No. 2,812,761 to Keefer, and in an article entitled Development of Flow Work Exchangers for Energy Recovery in Reverse Osmosis Plants, Research and Development Progress Report No. 680, April 1971, by Gilbert et al, U.S. Government Printing Office Stock No. 2400-0633. A current survey type article on the subject, "Office of Water Research and Technology Research Program on Energy Recovery Systems" was presented by M. R. Mattson and E. P. Easton, Jr. at the National Water Supply Improvement Association Conference in July 1980. With prior art arrangements, pumping means operated by the spent brine discharged from the reverse osmosis system are incapable of operating at a sufficiently high discharge pressure to feed fresh brine to the system without the use of some auxiliary booster pump which, of course, adds to the initial and maintenance costs of such systems.